1. Field of the Invention
The present invention relates to an optical pickup using a three beam tracking method. More particularly, the present invention relates to an optical pickup using a three beam tracking method wherein a light source emitting three separated laser beams is used.
2. Description of the Prior Arts
Data recording/reproducing apparatuses using optical disks such as a laser disk (LD) and a compact disk (CD) have been commercially available in recent years. In order to read out data from an optical disk, a laser beam is irradiated onto a data recording track (to be referred to as a "track" hereinafter), and data are reproduced based on the beam reflected by the track. When the tracks are helically formed on the optical disk, since the sectors of a single track are not equidistant from the center of the rotation of the disk, tracking (radial) control is necessary in the read mode to accurately irradiate the track with a laser beam. Even if the tracks are concentrically formed, sectors of a single track are not equidistant from the center of the rotation of the disk due to the eccentricity in the disk, and therefore, tracking control is essential. This tracking control has been conventionally performed by a push-pull method (one beam method) using single laser beam or a three beam method (out-trigger method) wherein the single laser beam is separated into three beams. However, the three beam method is more often used because it is more stable for the tilting of disk or the defect of disk than the one beam method. One example of this three beam tracking method is disclosed in U.S. Pat. No. 5,128,914 (issued to Kurata et al.).
As a typical conventional example of a tracking circuit, a lens tracking circuit maybe mentioned wherein an objective lens is moved in response to a tracking error signal derived from the beam reflected by or transmitted through an optical disk, e.g., one beam or three beams formed from a beam irradiated from the laser by the one beam method or three beam method. The objective lens is normally supported and fixed by a spring on an optical head housing. A tracking actuator is energized to move the lens for the tracking control. When the tracking actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
When the track eccentricity exceeds twenty or thirty microns, the objective lens deviates greatly from the mechanically neutral point, thereby combining an optical offset signal with the tracking error signal. The laser beam then traces a wrong track in response to the optical offset signal.
In order to eliminate the optical offset signal, a tracking system called a two-step servo system has been developed, as described in U.S. Pat. No. 4,761,773 (issued to Okada et al.). According to this system, the carriage and hence the optical pickup itself as well as the objective lens are moved to perform cooperative tracking.
Meanwhile, since the distance from the optical pickup to disk shifts minutely in the read mode in which the disk is rotated, it is difficult to correctly read data due to the shift, thus rendering focusing control essential. This focusing control has been conventionally performed by astigmatic method using astigmatism or a knife edge method.
As a typical conventional example of a focusing circuit, a lens focusing circuit may be mentioned wherein an objective lens is moved in response to a focusing error signal derived from laser beam which is irradiated from laser source and then reflected by or transmitted through an optical disk. The objective lens is normally the same lens that is used for the tracking control. A focusing actuator is energized to move the lens for the focusing control. When the focusing actuator is deenergized, the lens is held at a mechanically neutral point balanced by a spring force.
The conventional optical pickup of the three beam tracking method will be described in detail with reference to FIG. 1.
A light source 10 is provided with one laser diode to emit a laser beam. In the case of the three beam method, a grating 18 is provided at the front or the rear of a collimator lens 11, that is, between laser source 10 and collimator lens 11 or between collimator lens 11 and a beam splitter 12, which separates one beam into three beams. The laser beam irradiated from laser source 10 is changed into parallel beams by collimator lens 11. These parallel beams, in the three beam case, are separated into three beams by grating 18 and then pass through the beam splitter 12, a .lambda./4 plate 13, and an objective lens 14 to be incident upon the surface R of the disc D to form a beam spot of about 1 .mu.m.
Beam splitter 12 has two right-angled prisms of which oblique (45.degree. ) facets meet with each other. A polarizing film is formed on the oblique facet so that while ensuring the straight property of the incident beam, a part of the incident beam transmits through the prisms, and the other part is reflected from the polarizing film at an angle of 90.degree. together with an incident light. Additionally, .lambda./4 plate 13 serves to avoid interference of the incident beam and the reflective beam, and transforms a linear polarization into a circular polarization or inversely using double refraction phenomena.
The intensity of the light reflected from the disk D depends on existence of a pit thereof having recorded data. The recorded information is read out on the basis of the intensity of the reflected light. The reflected light is transformed into the parallel light while going through objective lens 14, polarized by 90.degree. at .lambda./4 plate 13, and then is incident onto beam splitter 12. In beam splitter 12, one part of the incident beam is reflected at 90.degree.. A converging lens 15 is placed at the optical path of the reflected light to converge the reflected light. The reflected light converged by converging lens 15 goes through cylindrical lens 16 (or knife edge) and then is received by a four or six partitioned light receiving element 17. The position errors of the pickup apparatus with respect to the disk D including a focusing error and a tracking error are detected from the image of the beam received on light receiving element 17, and focusing error signals and tracking error signals are generated according to these errors. A voice coil motor 19 as the objective lens actuator is actuated to move the objective lens in response to the error signals, so that focusing and tracking can be controlled. The information on the disk is reproduced on the basis of the intensity of the reflected light which is determined by pit P on the disc D.
According to the conventional optical pickup of the three beam tracking method, when the laser beam is irradiated from laser light source 10 having one laser diode, the one laser beam is split into three beams of a main beam 21, a first guide beam 21a and a second guide beam (21b) by grating 18. As shown in FIG. 2A, main beam 21 of three beams is irradiated to form a beam spot on disk D along the central part of the track. First and second guide beams 21a and 21b are irradiated on the upper and lower positions with respect to the position on which main beam 21 is irradiated, and on slightly deviated positions from the track center while being spotted on the track. As shown in FIG. 2B, the beams reflected from disk D are received by a main beam receiving part 17a and first and second guide beam receiving parts 17b and 17c of receiving element 17, respectively.
In order to improve the sensitivity with respect to the tracking error, it is necessary that grating 18 for separating one laser beam into three beams should be controlled so that the interval of the beam spots of the separated laser beams on the disk surface may be about 1/4 track pitch wherein the track pitch means a pitch between one track and an adjacent track on the disk.
However, according to the conventional optical pickup in which three beams are formed by the grating to perform the tracking as described above, in order that the phase difference of two guide beams is 180.degree., that is, two guide beams are positioned at the position of .+-.1/4 track pitch with respect to the main beam, controlling grating 18 with a high degree of precision is necessary. Furthermore, in order that at a tracking servo off mode the two guide beams are continuously maintained at the position of .+-.1/4 track pitch and thus the intensity of the tracking error signal is maintained to the maximum value, controlling grating 18 with a high degree of precision is necessary. When the azimuth angle is changed in the case of the pickup of a swing arm type, an angle between track line 20 and a line formed by first and second guide beams 21a and 21b changes continuously, and thereby an amplitude of the tracking error signal changes continuously, too. As a result, it is difficult to provide satisfactory tracking control.